Transparent, colorless low-titania β-quartz glass-ceramic material

ABSTRACT

The invention mainly relates to a transparent, essentially colorless/?β-quartz glass-ceramic material, the composition of which is free of As 2 O 3  and of Sb 2 O 3  and contains a specific combination of three nucleating agents: TiO 2 , ZrO 2  and SnO 2 ; TiO 2  being present in low quantity.

TECHNICAL FIELD

The present invention relates to glass-ceramic materials in general, precursor glasses therefore, articles comprising a glass-ceramic material, and process for making a glass-ceramic material. In particular, the present invention relates to glass-ceramic materials comprising β-quartz as the predominant crystalline phase that are essentially transparent and colorless in the visible spectrum, as well as precursor glass materials thereof, articles comprising the same and processes for making the same.

BACKGROUND

Transparent glass-ceramic materials with a low thermal expansion coefficient (CTE), which contain a solid solution of β-quartz as the principal crystalline phase, have been described in a number of publications, in particular by W. Hoeland and G. Beall, in “Glass-ceramic technology”, Am. Ceram. Soc., Westerville (2002), pages 88-96. Said glass-ceramic materials are generally obtained by heat treating a precursor glass (more conventionally a mixture of the constituents of said glass: a mineral charge, a precursor of such glass), the composition of which is of the LiO₂—Al₂O₃—SiO₂ (LAS) type. Said heat treatment includes a nucleation stage followed by a crystal growth stage.

The manufacture of articles made of β-quartz glass-ceramic conventionally includes the three principal successive steps: a first step of melting a batch, such as a mixture of raw starting materials and/or glass cullet, usually carried out between 1550° C. to 1750° C.; a second step of cooling and forming the molten glass obtained into desired shape; and a third step of crystallizing or ceramming the shaped cooled glass by a suitable heat treatment (including the phases of nucleation and crystal growth mentioned above).

Glass-ceramic materials comprising β-quartz solid solution as the predominant crystalline phase, having various degrees of transparency and being essentially colorless, were known. For example, U.S. Pat. Nos. 4,438,210; 5,591,682; 6,677,046; United States Patent Application Publication No. 2004/0198579; GB 2 159 154; EP 0 437 228; JP 2001-348250; DE 199 39 787 and WO 02 162 79; DE 101 10 225; and DE 199 07 038, are all concerned with transparent glass-ceramic materials.

Nonetheless, current commercial products of essentially transparent, essentially colorless glass-ceramic tend to have an undesirable color tint in the visible spectrum. There remains a need of a transparent, colorless glass-ceramic comprising β-quartz solid solution as the predominant crystalline phase.

SUMMARY

A first aspect of the present invention relates to a transparent, essentially colorless glass-ceramic material, comprising a β-quartz solid solution as the predominant crystalline phase, having a composition, expressed in terms of weight percentages on the basis of oxides, consisting essentially of:

SiO₂    >65-71 Al₂O₃      19-23 Li₂O       3-4 B₂O₃       0-1 TiO₂     0.3-<1.6 SnO₂   0.25-1.2 TiO₂ + SnO₂  <2 CeO₂      0-0.4 WO₃ + MoO₃      0-<1 CeO₂ + WO₃ + MoO₃      0-<1 Nb₂O₅      0-0.6 ZrO₂    >2-3.8 ZrO₂ + TiO₂ + SnO₂  >3.0-<4.8 MgO       0-2 ZnO       1-4 SrO       0-2 BaO       0-1.8 P₂O₅       0-3 K₂O + Na₂O       0-2.1 Gd₂O₃ + La₂O₃ + Ta₂O₅ + Y₂O₃       0-4 Nd₂O₃ + Er₂O₃       0-0.08 Fe₂O₃  <0.03; and and being free of arsenic oxide and antimony oxide, with the exception of unavoidable traces thereof.

In certain embodiments of the glass-ceramic material of the first aspect of the present invention, the glass-ceramic material has a composition, expressed in terms of weight percentages on an oxide basis, consisting essentially of:

SiO₂    67-70 Al₂O₃ 19.5-<22 Li₂O   3.2-3.8 B₂O₃     0-1 TiO₂   0.3-1.2 SnO₂   0.3-0.8 TiO₂ + SnO₂ <1 CeO₂     0-0.2 Nb₂O₅     0-0.2 ZrO₂   2.2-3.3 ZrO₂ + TiO₂ + SnO₂ >3.0-<4.4 MgO     0-1.5 ZnO   1.3-2.4 SrO     0-1.2 BaO     0-1.5 K₂O     0-1.3 Gd₂O₃ + La₂O₃ + Ta₂O₅ + Y₂O₃     0-2 Nd₂O₃ + Er₂O₃     0-0.06 Fe₂O₃ <0.02.

In certain embodiments of the glass-ceramic materials of the first aspect of the present invention, which may or may not be embodiments specifically described above, the composition of the glass-ceramic material is further free of halides, with the exception of unavoidable traces thereof.

In certain embodiments of the glass-ceramic materials of the first aspect of the present invention, which may or may not be embodiments specifically described above, the composition of the glass-ceramic material is further free of phosphates, with the exception of unavoidable traces thereof.

A second aspect of the present invention relates to an article made of a glass-ceramic material according to the first aspect of the present invention (including but not limited to the specific embodiments of the first aspect of the present invention described above), such as a cook plate, a cooking utensil, a plate of microwave oven, a fireplace window, a fire door or window, a viewing window for pyrolysis or catalysis furnaces, a lens item, an item of tableware, an architectural element or part of a ballistic protection.

A third aspect of the present invention relates to a lithium aluminosilicate glass, which is a precursor glass of a glass-ceramic material according to the first aspect of the present invention described above (including but not limited to those of the specific embodiments of the first aspect of the present invention described above). The composition of the glass material of the third aspect of the present invention corresponds to the composition of a glass-ceramic material of the first aspect of the present invention.

A fourth aspect of the present invention relates to a method for preparing a glass-ceramic material of the first aspect of the present invention, comprising the heat treatment of a lithium aluminosilicate glass, a precursor of said glass-ceramic material, or of a mineral charge, itself a precursor of such a lithium aluminosilicate glass, under conditions ensuring its ceramming, characterized in that said glass or said mineral charge has a composition which corresponds to that of a glass-ceramic material of the first aspect of the present invention.

In certain embodiments of the method of the fourth aspect of the present invention, the method comprises the following steps in sequence: (i) melting a lithium aluminosilicate glass, or a mineral charge, a precursor of such a glass, said glass or said charge containing an efficient, non-excessive quantity of at least one fining agent; followed by fining the molten glass obtained; (ii) cooling the fined, molten glass obtained and, simultaneously, forming it into desired shape for the intended article; (iii) ceramming said formed glass; characterized in that said glass or said mineral charge has a composition which corresponds to that of a glass-ceramic material according to the first aspect of the present invention (including but not limited to the specific embodiments of the first aspect of the present invention described above).

In certain embodiments of the method of the fourth aspect of the present invention, the method is characterized in that ceramming is implemented for a time of 150 min or less at a temperature of less than 1000° C., advantageously less than 950° C.

One or more embodiments of the various aspects of the present invention have one or more of the following advantages: an essentially colorless, highly transparent glass-ceramic material comprising β-quartz solid solution as the predominant crystalline phase can be made.

Additional embodiments of the invention will be set forth, in part, in the detailed description, and any claims which follow, and in part will be derived from the detailed description, or can be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates certain embodiments of the instant invention and together with the description, serves to explain, without limitation, the principles of the invention.

The accompanying drawing (FIGURE) shows the transmission curves of (i) a glass-ceramic material according to one embodiment of the present invention (Example 4 below); and (ii) a comparison example (Keralite®, a glass-ceramic material comprising TiO₂ described below).

DETAILED DESCRIPTION

The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various embodiments of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.

Unless otherwise indicated, all numbers such as those expressing weight percents of ingredients, dimensions, and values for certain physical properties such as coefficient of thermal expansion (CTE) used in the specification and claims are to be understood as being modified in all instances by the term “about.” It should also be understood that the precise numerical values used in the specification and claims form additional embodiments of the invention. Efforts have been made to ensure the accuracy of the numerical values disclosed in the Examples. Any measured numerical value, however, can inherently contain certain errors resulting from the standard deviation found in its respective measuring technique.

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “glass-ceramic material” includes embodiments having two or more such glass-ceramic materials unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

As used herein, a “wt %” or “weight percent” or “percent by weight” of a component, unless specifically stated to the contrary, is based on the total weight of the composition or article in which the component is included.

The present invention relates to the field of transparent, essentially colorless β-quartz glass-ceramic materials. The subject-matter of the invention is more particularly: (i) novel transparent, essentially colorless β-quartz glass-ceramic materials whose composition is devoid of As₂O₃ and Sb₂O₃, said composition containing a very specific combination of three nucleating agents: TiO₂, ZrO₂ and SnO₂; the TiO₂ content being low; (ii) articles made of said novel glass-ceramic materials; (iii) lithium aluminosilicate glasses, the precursors of said novel glass-ceramic materials, allowing the preparation of said novel, transparent, essentially colorless glass-ceramic materials, with short ceramming times, which is highly advantageous from an economic viewpoint; and (iv) methods for preparing said novel glass-ceramic materials and said articles made of said novel glass-ceramic materials.

To obtain the desired microstructure (comprising a solid β-quartz and β-eucryptite solution (simply called “β-quartz” in the remainder of this description) in a vitreous matrix), with optimization of said microstructure (optimization regarding the size and distribution of crystallites to obtain translucent, even transparent glass-ceramic materials with low thermal expansion coefficient (CTE)), usually efficient nucleating agents are used. Generally TiO₂ and/or ZrO₂ are used. TiO₂ is by far the most frequently used nucleating agent, since ZrO₂, when used alone, must be used in amounts close to the solubility limit (resulting in a higher melting temperature for the precursor glass, inhomogeneous nucleation, risks of devitrification during processing and/or residual ZrO₂-rich areas in the glass-ceramic material). Furthermore, ZrO₂ is a much less efficient nucleating agent than TiO₂. It requires much longer ceramming times.

To obtain transparent, “essentially colorless” glass-ceramic materials, the presence of coloring sites within said glass-ceramic materials, i.e. the presence of ions or ion pairs which on exposure to visible light may undergo electronic transitions, is to be avoided. However, it is to be noted that under “essentially colorless” is to be understood both “intrinsically essentially colorless” (due to the absence of ions or of ion pairs such as mentioned above) and “essentially colorless due to compensating coloring” by development of a complementary color within the material (see for example the teaching of U.S. Pat. No. 4,093,468). These notions are familiar to persons skilled in the art.

Though it appears the presence in a glass-ceramic of compounds imparting coloration could be avoided by avoiding or minimizing the introduction of said compounds or precursors thereof into the raw materials, the situation becomes more complex when certain necessary components can interact with a color-imparting species in the glass-ceramic material. For example, it is known that the presence of Fe₂O₃ alone (no TiO₂) up to contents of the order of 300 ppm in a glass-ceramic is generally not a concern as regards coloration. However, the joint presence of Fe₂O₃ and TiO₂ generates a characteristic yellowish tint. A number of commercial products which are otherwise known for their high transparency retain this yellowish tint, in particular those sold by the Applicant under the trade name KERALITE® (described in European patent application EP 0 437 228), those sold by Schott AG under the trade name ROBAX® and those sold by Nippon Electric Glass under the trade name NEOCERAM® N-0, because of the joint presence in their compositions of TiO₂ and Fe₂O₃. Treating the raw materials used to reduce the Fe₂O₃ content to below 150 ppm in particular is an expensive operation (an option mentioned in Japanese patent application JP 2001-348250) and it is seen above that TiO₂ is the best performing nucleating agent, allowing ceramization to occur on reasonable time scales. To solve the above-mentioned technical problem—obtaining transparent β-quartz glass-ceramic materials with no yellowish coloring—one possible approach appears to be managing without TiO₂, or at least minimizing the content thereof, when preparing these glass-ceramic materials.

It is also to be noted that after the first melting step in the method for preparing an article made of β-quartz glass-ceramic material as specified above, it is expedient to remove the gaseous inclusions as efficiently as possible from the mass of molten glass. For this purpose, at least one fining agent is used. To date, as fining agent, As₂O₃ and/or Sb₂O₃ have mostly been used (see above). The use of CeO₂, SnO₂ and other compounds such as halides has also been described. Since As₂O₃, the halides and Sb₂O₃ are advantageously excluded on account of their toxicity, said halides and Sb₂O₃ also being highly volatile, persons skilled in the art chiefly recommend the use of SnO₂. On the other hand, CeO₂ is known to generate a strong yellow coloring in the presence of TiO₂, and the inventors have evidenced the same problem subsequently to SnO₂ (and Nb₂O₅) interaction with TiO₂.

Consequently, it appeared to the inventors that in the composition of a transparent, essentially colorless glass-ceramic material, the presence of TiO₂ should advantageously be minimized, or even avoided, not only due to the interaction of TiO₂ with Fe₂O₃ but also due to the interaction of TiO₂ with non-toxic fining agents such as SnO₂, CeO₂ and Nb₂O₅.

Glass-ceramic materials, whose composition does not contain TiO₂, have been described (U.S. Pat. Nos. 3,252,811; 3,977,886; and 5,017,519). However, those skilled in the art know that the absence of TiO₂ results in long ceramming time, hence higher production costs and the presence of a greater amount of ZrO₂, which has the above-mentioned disadvantages.

The technical problem approached by said inventors was therefore to obtain transparent, essentially colorless β-quartz glass-ceramic materials and glass-ceramic material articles (with a low thermal expansion coefficient), from precursor glasses with low TiO₂ content and free of undesirable fining agents (As₂O₃ and Sb₂O₃); and with reasonable cerammization times (unexpectedly this proved to be possible within 150 min or even less).

With reference to said technical problem, the inventors identified a narrow family of glass-ceramic materials of particular interest, whose composition concomitantly contains TiO₂, ZrO₂ and SnO₂ (and optionally CeO₂ and/or WO₃ and/or MoO₃ and/or Nb₂O₅).

According to its first aspect, the present invention therefore relates to a transparent, essentially colorless glass-ceramic material containing a solid β-quartz solution as main crystalline phase, having a composition, expressed in terms of weight percentages on an oxide basis, consisting essentially of:

SiO₂  >65-71 Al₂O₃    19-23 Li₂O     3-4 B₂O₃     0-1 TiO₂  0.3-<1.6 SnO₂ 0.25-1.2 TiO₂ + SnO₂ <2, advantageously <1.8 CeO₂     0-0.4 WO₃ + MoO₃     0-<1 CeO₂ + WO₃ + MoO₃     0-<1 Nb₂O₅     0-0.6 ZrO₂   >2-3.8, advantageously 2.2-3.8 ZrO₂ + TiO₂ + SnO₂ >3.0-<4.8 MgO     0-2 ZnO     1-4 SrO     0-2 BaO     0-1.8 P₂O₅     0-3 K₂O + Na₂O     0-2.1, advantageously 0-1.5 Gd₂O₃ + La₂O₃ + Ta₂O₅ + Y₂O₃     0-4 Nd₂O₃ + Er₂O₃     0-0.08 Fe₂O₃ <0.03; and, with the exception of unavoidable traces, free of arsenic oxide and antimony oxide.

The terms “transparent” and “essentially colorless” are familiar to those skilled in the art. They are quantified below (the values indicated remain, however, magnitudes and should not be interpreted strictly): samples of the glass-ceramic materials according to the invention, 3 mm thick, generally have the following values given below for the L* (lightness), a* and b* (color coordinates) parameters in the color space laid down by “CIE 1976 Lab” (International Commission on Illumination, 1976), measured using the standard C illuminator:

-   -   L*>90     -   −2<a*<2     -   −2<b*<12.

Persons skilled in the art generally acknowledge that an “L*” value of more than 90 is required for high transparency, and an “a*” value of less than 2 is required for a slight yellowish tint. It is observed that a “b*” value of more than 12 is generally associated with an opalescent appearance.

To quantify transparency, the following were measured on samples of the same type (3 mm thick): the lowest wavelengths in which transmission T respectively exceeds 60 and 10%, the difference being calculated to assess opalescence. Results were generally:

-   -   T₁₀≦360 nm     -   T₆₀≦435 nm, and     -   T₆₀−T₁₀<75 nm.

Also, the thermal expansion coefficient (CTE) of the glass-ceramic materials according to the invention (measured from 25° C. to 700° C.) generally lies from −10×10⁻⁷K⁻¹ to +15×10⁻⁷ K⁻¹.

It was indicated that the solid β-quartz solution is present as main crystalline phase. Within the glass-ceramic materials of the invention the residual vitreous phase generally represents less than 35 weight %, and the solid β-quartz solution represents at least 65 weight %, generally from 70 to 85 weight %, of the crystallized fraction. With reference to the crystalline phase of the glass-ceramic materials according to the invention, it can be specified as follows but in no way limited thereto. Generally, said crystalline phase chiefly consists of at least 80 wt. % solid β-quartz or β-eucryptite solution. Said crystalline phase also generally contains minor quantities of secondary crystalline phases, such as phases of srilankite, cubic ZrO₂, rutile, possibly in a solid solution with SnO₂. Said crystalline phase may also contain tertiary crystalline phases such as a solid solution of β-spodumene (less than 3 wt. %) or others (less than 5 wt. %).

The crystallite size is generally less than 70 nm, preferably 60 nm or less.

It was indicated that the composition “essentially consists of” the given list of compounds (oxides). This means that in the glass-ceramic materials according to the invention, the sum of the listed compounds (oxides) represents at least 95%, generally at least 98 weight %. It cannot be fully excluded that in said glass-ceramic materials other compounds may be found in low quantities.

With reference to the weight composition of the glass-ceramic materials according to the invention, it can be specified as follows, although in no way limited thereto.

(1) The glass-ceramic materials under consideration are of LAS type. They contain Li₂O, Al₂O₃ and SiO₂ as essential constituents of the solid β-quartz solution, which gives them their transparency and low thermal expansion coefficient (CTE). The indicated ranges for said essential constituents are narrow. Hence it was determined that: (i) the SiO₂ content is more than 65% and no more than 71% to obtain results of interest in terms of characteristics of the end product (high JO transparency and low thermal expansion coefficient (CTE)) and in terms of the method for obtaining said end product (melting procedure and ceramming time). The SiO₂ content advantageously lies from 67 to 70%; (ii) the Al₂O₃ content is limited to from 19 to 23%, advantageously from 19.5 and less than 22%. If said Al₂O₃ content is not high enough (<19%), the transparency of the end product decreases and ceramming becomes too slow. If said Al₂O₃ content is too high (>23%), melting and ceramming are difficult to implement, and devitrification phenomena can be seen to occur when forming said glass; (iii) the Li₂O content is limited to from 3 to 4%, advantageously from 3.2 to 3.8%. A minimum of 3.2% Li₂O is generally necessary to obtain a transparent glass-ceramic material with low thermal expansion coefficient (CTE) and to minimize ceramming time. If the Li₂O content is too high, devitrification phenomena can be seen to occur.

(2) The glass-ceramic materials according to the invention may contain borates. In particular B₂O₃ may be beneficially used to dissolve ZrO₂ and to lower viscosity on melting. However, B₂O₃ is known to facilitate phase separation and may be responsible for opalescence, for the presence of large crystals and β-spodumene. Therefore the glass-ceramic materials according to the invention do not contain more than 1 weight % B₂O₃.

(3) As nucleating agents the glass-ceramic materials according to the invention contain: TiO₂ (in limited quantities), ZrO₂ and SnO₂. The amount of these three components are typically:

TiO₂    0.3-<1.6 SnO₂   0.25-1.2 TiO₂ + SnO₂ <2, advantageously <1.8 ZrO₂    >2-3.8, advantageously 2.2-3.8 ZrO₂ + TiO₂ + SnO₂  >3.0-<4.8,

These amounts of these components enable the ceramming (TiO₂+SnO₂+ZrO₂>3%, SnO₂ also acting as fining agent); and, most unexpectedly, within short time periods (150 min, or less: see below). It also enables the onset of the yellowish tint to be minimized or even avoided (with the indicated Fe contents) due to the low TiO₂ (<1.6%), SnO₂ (≦1.2%), TiO₂+SnO₂ (<2%, advantageously <1.8%) amounts. In addition, the melting process is fully controlled and the onset of devitrification phenomena is minimized (ZrO₂+TiO₂+SnO₂<4.8%).

In certain embodiments, it is desired that:

TiO₂   0.3-1.2 SnO₂   0.3-0.8 TiO₂ + SnO₂ <1 ZrO₂   2.2-3.3 ZrO₂ + TiO₂ + SnO₂ >3.0-<4.4.

Generally, the values in the composition of the glass-ceramic materials according to the invention are most advantageously: TiO₂ 0.6%-0.8 wt % and/or 3.5%<ZrO₂+TiO₂+SnO₂<4.4%.

(4) SnO₂, in addition to its function as nucleating agent, also ensures the function of fining agent. It is recalled that the composition of the glass-ceramic materials according to the invention is free of arsenic oxide and antimony oxide. It was seen above that the content of SnO₂ is at least 0.25% (with reference therefore to its efficiency as nucleating agent and fining agent) and is no more than 1.2%, with: TiO₂+SnO₂<2, advantageously <1.8%, and 3.0%<ZrO₂+TiO₂+SnO₂<4.8% (with reference to the problems of coloring, nucleation, implementation of the melting process and devitrification). The advantageous and very advantageous conditions of use of SnO₂ have already been indicated above.

(5) CeO₂, WO₃, MoO₃ and Nb₂O₅ may be used, either alone or in combination, as fining agent. The sum of oxides of tungsten and/or molybdenum, expressed as WO₃+MoO₃, is limited to less than 1 wt. % for the following reasons: while both constituents in limited quantity can be used to assist fining of the melt, if their total amount exceeds 1 wt. %, a large number of small bubbles may form in the glass melt at temperatures between 1550° C. and 1750° C., and those bubbles are very difficult to remove. Thus, used in too large quantities, the effect of WO₃ and MO₃ may be rendered negative. Additionally, both constituents and particularly MoO₃, if used in too large quantities, may generate different types of colorations in the final glass-ceramic. The use of CeO₂+WO₃+MoO₃ is limited to less than 1%. Beyond this value, the onset of a yellowish tint is observed. Advantageously, WO₃ and MoO₃ are not present and the contents of CeO₂ and Nb₂O₅ are each limited to 0.2%.

(6) ZnO and the alkaline-earth oxides of the MgO, SrO and BaO group are used to improve the melting properties, to stabilize the vitreous phase and to influence the microstructure of the glass-ceramic material. It is known that SrO and BaO generally remain in the vitreous phase, while Mg is rather incorporated in the solid solution. ZnO also enables the thermal expansion coefficient (CTE) to be lowered, while MgO, BaO and SrO increase this coefficient. The heavier elements such as Ba and Sr have an impact on the refractive index of the vitreous phase and also on turbidity. It was indicated that the glass-ceramic materials according to the invention contain 0 to 2% MgO, 1% to 4% ZnO, 0 to 2% SrO and 0 to 1.8% BaO. Their MgO content advantageously lies from 0 and 1.5%; their ZnO content from 1.3% to 2.4%; their SrO content from 0 to 1.2%; and their BaO content from 0 to 1.5%.

(7) The glass-ceramic materials according to the invention may also contain 0 to 3% P₂O₅. Advantageously they do not contain any phosphates: thereby homogeneity and transparency can so be optimized.

(8) The glass-ceramic materials according to the invention may also contain 0 to 2.1%, advantageously 0 to 1.5%, alkaline oxides other than Li₂O, e.g., Na₂O and K₂O. Preferably Na₂O is not present. Preferably, K₂O is present alone (at a content of from 0 to 1.3%). After ceramming, the alkaline ions remain in the vitreous phase. They increase thermal expansion and may therefore be used to compensate for too negative thermal expansion coefficient (CTE) values. They can also be used to lower the melting temperature and to increase ZrO₂ dissolution, i.e. to simplify the implementation of the method. If they are used in too great a quantity, thermal expansion will be too strong and nucleation may become difficult to control.

(9) The glass-ceramic materials according to the invention may also contain up to 4% oxides, such as Gd₂O₃, La₂O₃, Ta₂O₅ and Y₂O₃ (this list not being truly exhaustive). Said oxides can allow increased transparency and optical appearance of the glass-ceramic, by increasing the refractive index of the residual vitreous phase, without coloring said glass-ceramic. If they are used in too great a quantity, thermal expansion increases, the refractive index may become too high and the melting step difficult to conduct. Advantageously, the glass-ceramic materials according to the invention do not contain more than 2 weight % of such oxides.

(10) The presence of colorant(s) complementary to the yellow in the glass-ceramic materials according to the invention is not excluded. This aims to improve the desired object: the elimination of any yellowish tinge (by compensation). In particular, use may be made of Nd₂O₃ and/or Er₂O₃. Nd₂O₃ and Er₂O₃ must be used in limited quantities, from 0 to 0.08%, advantageously from 0 to 0.06%, further advantageously from 0 to 0.04%. For example, if too much Nd₂O₃ is used, a bluish tint is observed; if too much Er₂O₃ is used, a pink tint is observed.

(11) Finally, it was specified that the Fe₂O₃ content in the glass-ceramic materials according to the invention was less than 300 ppm. Obviously, Fe₂O₃ is not voluntarily added as a constitutive glass ingredient. If it is present, this is because it is a common impurity in the raw materials used. Within the context of the invention, Fe₂O₃ is only likely to interfere with a limited quantity of TiO₂. Generally, it is obviously preferred to minimize the presence of iron but if, for this purpose, the raw materials used have to be purified, then this would often prove to be too costly. Also in some cases the presence of Fe₂O₃ can prove to be of interest with respect to melting and fining. Generally, the glass-ceramic materials according to the invention contain 100 to 250 ppm Fe₂O₃. Advantageously, the glass-ceramic materials according to the invention contain less than 200 ppm Fe₂O₃.

The glass-ceramic materials according to the invention whose composition has just been described above, are also characteristically free of arsenic oxide and antimony oxide, with the exception of unavoidable traces thereof: in this way the use of these undesirable products is avoided.

None of these compounds is therefore voluntarily added as a raw material when preparing the glass-ceramic materials according to the invention.

Quite unexpectedly, it was possible to dispense with the action of said compounds and to limit the TiO₂ content in order to prepare glass-ceramic materials meeting the specifications set forth in the introduction hereto (more particularly: transparent, essentially colorless β-quartz glass-ceramic materials whose ceramming treatment may last less than 150 min).

The advantageous ranges indicated above are to be considered independently of each other, and also in combination with each other.

The glass-ceramic materials according to the invention advantageously have the following weight composition [i.e. their composition expressed in oxide weight percent essentially consists (in the meaning specified above)] of:

SiO₂    67-70 Al₂O₃ 19.5-<22 Li₂O   3.2-3.8 B₂O₃     0-1 TiO₂   0.3-1.2 SnO₂   0.3-0.8 TiO₂ + SnO₂ <1 CeO₂     0-0.2 Nb₂O₅     0-0.2 ZrO₂   2.2-3.3 ZrO₂ + TiO₂ + SnO₂ >3.0-<4.4 MgO     0-1.5 ZnO   1.3-2.4 SrO     0-1.2 BaO     0-1.5 K₂O     0-1.3 Gd₂O₃ + La₂O₃ + Ta₂O₅ + Y₂O₃     0-2 Nd₂O₃ + Er₂O₃     0-0.06 Fe₂O₃ <0.02.

Advantageously, the composition of the glass-ceramic materials according to the invention is also free of halides, with the exception of unavoidable traces. Mention was made above of the problem related to the use of halides (corrosion, pollution). Advantageously therefore, no halide is voluntarily added as a raw material when preparing the glass-ceramic materials according to the invention.

Advantageously, the composition of the glass-ceramic materials according to the invention is free of phosphates, with the exception of unavoidable traces (see above). The presence of phosphates particularly facilitates opalescence. Therefore, advantageously no phosphate is voluntarily added as a raw material when preparing the glass-ceramic materials according to the invention.

Most advantageously, the composition of the glass-ceramic materials according to the invention is also free of halides and phosphates, with the exception of unavoidable traces thereof.

According to its second subject-matter, the present invention relates to articles made of glass-ceramic materials such as described above. Said articles may consist for example in cook plates, cooking utensils, plates for microwave ovens, fireplace windows, fire doors or fire windows, viewing windows for pyrolysis or catalysis furnaces, lens items, tableware items, architectural elements or parts of ballistic protections.

According to its third subject-matter, the present invention relates to lithium aluminosilicate glasses, the precursors of the glass-ceramic materials according to the invention such as described above. The lithium aluminosilicate glasses with the above-indicated compositions for the glass-ceramic materials according to the invention are effectively novel.

According to its fourth subject-matter, the present invention relates to a method for preparing a glass-ceramic material according to the invention such as described above. Conventionally, said method comprises thermal treatment of a lithium aluminosilicate glass, a precursor of said glass-ceramic material, or of a mineral charge, itself a precursor of such a lithium aluminosilicate glass, under conditions ensuring its ceramming. Said ceramming treatment is known per se.

Characteristically, according to the invention, the method is carried out with a glass or a mineral charge having a weight composition corresponding to that of a glass-ceramic material according to the invention such as specified above.

According to its fifth subject-matter, the present invention relates to a method for manufacturing an article made of a glass-ceramic material according to the invention. Conventionally, said method comprises the following three successive steps: (a) melting a lithium aluminosilicate glass, or a mineral charge, a precursor of such a glass, said glass or said charge containing an efficient, non-excessive amount of at least one fining agent; followed by fining the molten glass obtained; (b) cooling the fined, molten glass obtained and simultaneously forming it to desired shape for the intended article; and (c) ceramming said formed glass.

Characteristically, according to the invention, said glass or said mineral charge under consideration is present in a weight composition which corresponds to that of a glass-ceramic material according to the invention, such as specified above.

The above-mentioned forming (shaping) advantageously consists in rolling between rollers to obtain sheets.

The glass under consideration can be cerammed in 150 min or less.

The ceramming of said formed glass is advantageously conducted for a period of 150 min or less at a temperature of less than 1000° C., advantageously less than 950° C. It quite unexpectedly proved possible to obtain the glass-ceramic material according to the invention with such short ceramming times.

The ceramming time mentioned above corresponds to the time lapsed from the temperature of 650° C. to the maximum ceramming temperature (less than 1000° C.), advantageously from 650° C. to less than 950° C.; said ceramming time corresponds to the nucleation and crystal growth phases.

Said ceramming time does not include the time needed to reach the temperature of 650° C. or the cooling time from the maximum temperature.

It was indicated above that said ceramming time may be 150 min or less. It may even be 120 min or less, even 90 min or less. It most unexpectedly proved possible to obtain the glass-ceramic material according to the invention within these short ceramming times.

As variations of advantageous embodiments of the method according to the invention, the temperature of 650° C. is reached in less than one hour (starting from the temperature of the formed product), most advantageously in less than 30 min; and/or the glass-ceramic material obtained is cooled by at least 40° C. from its maximum ceramming temperature in less than 10 min.

Said invention is illustrated below with the following examples and the appended drawing.

EXAMPLES

To produce 1 kg batches of precursor glass, the raw materials in the proportions (expressed in oxides) given in the first part of Table 1 below were carefully mixed.

The mixtures were placed in platinum crucibles for melting. The filled crucibles were placed in a pre-heated furnace at 1400° C. They underwent the following melting cycle (i) rising temperature up to 1650° C. at a heating rate of 2° C./min; and (ii) holding this temperature of 1650° C. for 12 hours.

The crucibles were then removed from the furnace and the molten glass was poured onto a pre-heated steel plate. It was rolled to a thickness of 4 mm. Approximately 20 cm×30 cm glass plates were obtained. They were annealed at 650° C. for 1 h and then cooled gently.

The glass plates obtained were generally very transparent.

They were subjected to a ceramming (crystallization=nucleation+crystal growth) treatment such as specified below. Said glass plates were heated to 650° C. at a heating rate of 30° C./min. They were then heated to 820° C. in 40 min and held at the temperature of 820° C. for 10 min. They were finally heated from said temperature of 820° C. to 900° C. at a heating rate of 10° C./min and held at this temperature of 900° C. for 15 min.

The glass-ceramic materials obtained had the properties indicated in the second part of said Table 1.

The indicated transmissions T were measured on 3 mm thick glass-ceramic material samples. The T₁₀ and T₆₀ values (expressed in nm) correspond to the lowest wavelengths in which transmission T respectively exceeds 10 and 60%; their difference is indicative of opalescence.

The color points indicated as L*, a* and b* are those mentioned in the introduction hereto (lightness and color coordinates in the “CIE/1976 Lab” space, measured using standard illuminator C). They were assessed on 3 mm thick samples.

The thermal expansion coefficient (CTE) was measured by horizontal dilatometry (25° C.-700° C.).

The size of the crystals (generated on ceramming) was determined using a conventional X-ray diffraction technique on polished glass-ceramic material samples (Ø=32 mm; thickness 3 mm). Said size was calculated from diffractograms (X-ray) using Rietveld analysis known to those skilled in the art. The indicated values are rounded up or down to the closest tenth (e.g. 34 corresponds to 30 and 57 to 60).

Considering the information contained in said Table 1, the advantage of the present invention is confirmed.

Examples C1 to C5 are comparative examples.

The glass-ceramic material in example C1 is slightly opalescent (T₆₀−T₁₀ value is very high). Its TiO₂ content is too low. Its lightness L* is therefore too weak. Its crystal size is not optimized.

The glass-ceramic material in example C2 is also slightly opalescent (T₆₀−T₁₀ value is still high). Its SnO₂ content is too low. Its crystal size is not optimized.

The “glass-ceramic material” in example C3 is not cerammed, since the ZrO₂ content is too low, and since, more generally, the nucleating agent content is too low (TiO₂+ZrO₂+SnO₂: 2.9 wt. %).

The glass-ceramic material in example C4 is slightly yellow (which is confirmed by the “a*” color coordinate and T₁₀ values), in particular due to the cumulated TiO₂ (1.6%) and SnO₂ (0.4%) contents: TiO₂+SnO₂: 2%.

The glass-ceramic material in example C5 is slightly opaque. Its B₂O₃ content is too high. Consequently large crystals have developed. Their “harmful” presence shows as said slight opalescence.

Examples 1 to 9 illustrate the invention.

The glass-ceramic material in example 1 contains a relatively high quantity of SnO₂ (0.8%). Therefore, relatively high T₆₀, T₆₀−T₁₀ difference and “a*” color coordinate values are observed. However, the result is acceptable.

The glass-ceramic materials in examples 2 and 4 are particularly preferred.

They show very low T₁₀ values, low T₆₀−T₁₀ values, low thermal expansion coefficients (CTE) and a crystal size of particular interest. These good results are obtained with a short ceramming time. This—short—ceramming time is the same (73 min) for all the examples (see above).

The glass-ceramic material in example 3 has a low TiO₂ content. Unexpectedly, with such a low TiO₂ content, ceramming could be conducted and could even be conducted within the indicated time period. However, with such T₆₀−T₁₀ and b* color coordinate values, a certain tendency towards opalescence is to be noted. The result nonetheless remains very acceptable.

Example 5 illustrates a variant of a glass-ceramic material according to the invention whose composition is devoid of MgO. The results in terms of color and transmission are excellent. Regarding said transmission, said excellent results can be explained by the crystal size (very small). However, the absence of MgO has an unfavorable impact on the thermal expansion coefficient (CTE). The overall result remains very acceptable.

Example 6 illustrates another variant of a glass-ceramic material according to the invention, whose composition is devoid of MgO. Due to the presence of Ta₂O₅ the thermal expansion coefficient (CTE) is higher. Similarly, the b* color coordinate value is close to the “tolerable” limit (12).

The glass-ceramic material in example 7 contains a small quantity of B₂O₃. Said small quantity is not detrimental to transparency. This example 7 is to be considered in parallel with comparative example C5. With said small quantity of B₂O₅, the crystal size is in the announced preferred range. The glass-ceramic material is highly transparent, colorless and has a thermal expansion coefficient (CTE) such as announced (of interest).

Examples 8 and 9 illustrate in more detail the roles of BaO and MgO. Glass-ceramic materials that satisfy the required specifications can be obtained from precursors glasses that are free of BaO, and also free of MgO. K₂O may be used to replace one or the other or both. However, in the same time or independently, it may be necessary to limit the amount of ZrO₂ (in order to prevent devitrification) and to compensate this by increasing the amount of TiO₂. Then, highly transparent products are derived. However, they possess a somewhat stronger (but acceptable) yellow tint that certain other embodiments of the invention (e.g. examples 1-5).

The accompanying drawing shows the transmission curves (transmission, expressed as a percentage, as a function of wavelength, expressed in nanometers) of two 3 mm (millimeter) thick glass-ceramic samples. The samples were prepared by cutting 32 mm diameter disks from glass-ceramic plates. The disks (4 mm thick) were then polished on both faces to a thickness of 3 mm. One sample was the material of Example 4 (“4” in the drawing) and the other was a comparative glass-ceramic material (“C” in the drawing, which is Keralite®, a TiO₂-containing glass-ceramic material disclosed in EP 0 437 228).

It should also be understood that while the present invention has been described in detail with respect to certain illustrative and specific embodiments thereof, it should not be considered limited to such, as numerous modifications are possible without departing from the broad scope of the present invention as defined in the appended claims.

TABLE 1 Examples C1 C2 C3 C4 C5 1 2 3 4 5 6 7 8 9 Composition (%) SiO₂ 68.4 68.6 69.3 68.4 67.9 68.1 68.6 68.4 68.4 67.0 66.1 67.9 67.4 67.5 Al₂O₃ 20.2 20.2 20.7 20.2 19.8 20.1 20.2 20.2 20.2 20.1 19.9 19.9 20.0 19.9 Li₂O 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.4 3.5 3.6 3.5 MgO 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 ZnO 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 2.0 2.0 1.6 1.8 1.8 TiO₂ 0.2 0.6 0.6 1.6 1.2 0.6 0.6 0.4 1.2 1.4 1.4 1.2 1.5 1.4 ZrO₂ 3.7 3.3 1.9 2.3 2.7 3.3 3.1 3.5 2.7 2.5 2.4 2.7 2.2 2.1 BaO 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1.4 0.8 0.8 SnO₂ 0.4 0.2 0.4 0.4 0.4 0.8 0.4 0.4 0.4 0.3 0.3 0.4 0.4 0.4 SrO 1.0 1.0 1.6 1.4 K₂O 0.8 0.8 1.5 2.0 Ta₂O₅ 1.9 B₂O₃ 1.2 0.8 Properties after ceramic Transmission T₁₀ (nm) 324 329 302 362 350 347 331 327 344 342 350 344 349 347 T₆₀ (nm) 447 406 332 395 420 414 372 392 383 373 418 382 389 386 T₆₀-T₁₀ (nm) 123 77 30 33 70 67 41 65 39 31 68 38 40 39 Appearance^(#) SOP SOP VTR SYW SOP VTR VTR VTR VTR VTR VTR VTR VTR VTR Color L* 89.8 91.8 96.6 95.1 93.2 91.1 95.4 92.4 95.6 95.0 91.4 95.3 95.2 95.1 a* −0.7 −0.6 −0.2 −1.4 −1.6 −0.7 −0.5 −0.6 −0.7 −0.6 −1.3 −0.8 −1.0 −0.8 b* 12.6 8.8 0.5 4.6 9.0 7.8 3.2 7.1 3.0 3.1 11.6 3.6 4.0 4.2 CTE 1.2 −0.1 36.8 −1.1 0.8 −0.3 −1.6 −1.3 −1.3 −2.1 −3.1 −2.0 0.4 1.8 (25° C.-700° C.) (10⁻⁷ K⁻¹) Size of β-quartz 60 60 NCR 40 70 40 50 50 30 20 40 50 30 30 crystals (nm^()##) ^(#)SOP: slightly opalescent; VTR: very transparent; SYW: slightly yellow; ^(##)NCR: Not crystallized. 

1. A transparent, essentially colorless glass-ceramic material, comprising a β-quartz solid solution as a predominant crystalline phase, having a composition, expressed in terms of weight percentages on a basis of oxides, consisting essentially of: SiO₂  >65-71 Al₂O₃    19-23 Li₂O     3-4 B₂O₃   ≧0-1 TiO₂   0.3-<1.6 SnO₂ 0.25-1.2 CeO₂     0-0.4 WO₃ + MoO₃     0-<1 CeO₂ + WO₃ + MoO₃     0-<1 Nb₂O₅     0-0.6 ZrO₂   2.2-3.8 ZrO₂ + TiO₂ + SnO₂ >3.0-<4.8 MgO     0-2 ZnO     1-4 SrO     0-2 BaO     0-1.8 P₂O₅     0-3 K₂O + Na₂O     0-1.5 Gd₂O₃ + La₂O₃ + Ta₂O₅ + Y₂O₃     0-4 Nd₂O₃ + Er₂O₃     0-0.08 Fe₂O₃ <0.03; and

free of arsenic oxide and antimony oxide, with the exception of unavoidable traces thereof and further comprising an amount of B₂O₃ effective to dissolve ZrO₂.
 2. The glass-ceramic material according to claim 1, having a composition, expressed in terms of weight percentages on an oxide basis, consisting essentially of: SiO₂   67-70 Al₂O₃ 19.5-<22 Li₂O   3.2-3.8 B₂O₃   ≧0-1 TiO₂   0.3-1.2 SnO₂   0.3-0.8 CeO₂     0-0.2 Nb₂O₅     0-0.2 ZrO₂   2.2-3.3 ZrO₂ + TiO₂ + SnO₂ >3.0-<4.4 MgO     0-1.5 ZnO     1.3-2.4 SrO      0-1.2 BaO      0-1.5 K₂O      0-1.3 Gd₂O₃ + La₂O₃ + Ta₂O₅ + Y₂O₃     0-2 Nd₂O₃ + Er₂O₃     0-0.06 Fe₂O₃ <0.02.


3. The glass-ceramic material according to claim 1, the composition of which is further free of halides, with the exception of unavoidable traces thereof.
 4. The glass-ceramic material according to claim 1, the composition of which is further free of phosphates, with the exception of unavoidable traces thereof.
 5. An article made of a glass-ceramic material according to claim 1, wherein said article is selected from the group consisting of a cook plate, a cooking utensil, a plate of microwave oven, a fireplace window, a fire door or window, a view window for pyrolysis or catalysis furnace, a lens item, an item of tableware, an architectural element and part of a ballistic protection.
 6. The glass-ceramic material according to claim 1, comprising about 0.8 wt. % B₂O₃.
 7. The glass-ceramic material according to claim 1, further comprising an effective amount of a fining agent selected from the group consisting of CeO₂ and Nb₂O₅.
 8. The glass-ceramic material according to claim 1, further comprising an effective amount of a refractive index increasing oxide selected from the group consisting of Gd₂O₃, La₂O₃, Ta₂O₅ and Y₂O₃. 